Whatever next is a pretty cool book, it's hard to keep it contained to a bag
You could include
the moon or a space book to compliment the theme & provide children the opportunity to choose or change the story direction.
Possibly add in a few natural fruits and vegetables to balance out the choccy biscuits & honey/syrup .. weather, transport, day & night symbols could also be good.
Have fun with it & any picnics you take part in xx